1. Field of the Invention
This invention relates to an F-M demodulator circuit and, in particular, to a phase detector type of demodulator that may be readily fabricated in integrated circuit configuration.
2. The Prior Art
Generally, conventional F-M demodulator circuits of the type that find application in video tape recording devices have usually been of either the pulse counter type, the delay line type, or the phase detector type.
In the pulse counter type of F-M demodulator circuit, a frequency modulated signal is usually transformed into a rectangular wave by, for example, a limiting circuit, and the rectangular wave is then differentiated to derive a pulse signal having a period representative of the frequency of the F-M signal. An output signal representing the frequency variations may be derived by supplying the resultant pulse signal to an integrating circuit, such as a low pass filter, via a bidirectional rectifying circuit.
Unfortunately, this pulse counter type of demodulator suffers from the disadvantage of a limited frequency response due to the differentiating circuit. As a result, the derived output signal will have a linear relationship with respect to the frequency variations of the F-M signal over a wide range of frequencies. Another disadvantage is that an error voltage might be superposed onto the output signal if the frequency modulated signal suffers from amplitude variations. A still further disadvantage is that the differentiating circuit generally is not readily adaptable for integrated circuit fabrication.
In the delay line type of F-M demodulator circuit, the frequency modulated signal is generally applied to a mixer circuit through a direct conducting channel and through a time-delay channel including the delay line. The mixer circuit is adapted to respond to the signals applied thereto to produce an output pulse signal having a duty ratio determined by the phase difference between the signals applied to the mixer circuit. A demodulated output signal is then obtained by integrating the produced output pulse signal.
However, the delay line type of demodulator generally suffers from many of the same disadvantages as the pulse counter type of demodulator. That is, the limited frequency response of the delay line generally limits the frequency range over which the demodulated output signal has a linear relationship with respect to the frequency variations of the F-M signal. Also, amplitude variations in the frequency modulated signal often induce error voltages in the output signal. Additionally, the delay line is generally not adapted for integrated circuit configuration.